1. Field of the Invention
The invention is concerned with multimode fibers for use as transmission lines in communications systems operating in the visible or near-visible spectra. Fibers of concern include a core region with decreasing refractive index from the center of the core in a radial direction. The purpose of the gradient is to compensate for the differing mode-dependent group velocities so as to lessen pulse spreading due to this mechanism.
2. Description of the Prior Art
Optical communications systems under general study popularly depend on a glass fiber transmission guide. A form under widespread investigation designed for use over medium and long distances utilizes a guide of sufficiently large dimension to accommodate many modes in addition to the fundamental. Multimode transmission guides have the advantage of energy conservation of particular interest for a particular class of radiation sources now available. An early recognized disadvantage of multimode transmission is the mode-dependent group velocity resulting in increasing residence time for higher order modes. The result of this "mode dispersion" phenomenon is to lessen the usable bandwidth of the system. This is most easily visualized for a pulse system in which the pulse broadens during transmission due to the varying differing group velocities of the many modes entering into its composition. The ultimate bandwidth limitation coincides with sufficient distance to result in smearing of successive pulses (or in the smearing of successive information bits in analog systems--e.g., the disappearance of amplitude variations in an AM system).
Workers have recognized for some time that modal dispersion may be lessened by providing a refractive index variation such as to compensate for mode-dependent group velocity variations. In simple terms, this approach simply provides a lower index and, therefore, more rapid medium in the outer sections of the core through which a statistically greater fraction of the higher mode energy is transmitted.
For some time it was believed that a parabolically varying index would accomplish minimization of mode dispersion. This initially simple concept has evolved into a somewhat more complex gradient function which takes into account the previously ignored dispersive effect due to the index-modifying dopant introduced to accomplish the desired variation. This has culminated in a paper by Robert Olshansky and Donald B. Keck published at page TuC5-1 et seq. of the technical digest Optical Fiber Transmission, (a digest of technical papers presented at the Topical Meeting an Optical Fiber Transmission. January 7-9, 1975, Williamsburg, Virginia).
It was early recognized that minimum mode dispersion coincides with gradient defined by the equation: EQU n = n.sub.1 [1-2.DELTA.(r/a).sup..alpha.].sup.1/2, 1
(see 52, Bell System Technical Journal, p. 1563 (1973)) in which n.sub.1 is the index maximum, r is the distance from the fiber axis, a is the core radius, .DELTA. is the relative index difference between core center and cladding, .alpha. is a power law exponent which characterizes the profile.
In accordance with Olshansky and Keck, supra., .alpha. is defined as: EQU .alpha. = 2+y-.DELTA. (4+y)(3+y)/(5+2y) 2
in which y is = -2n.sub. 1 /N.sub.1 .lambda./.DELTA. d.DELTA. /d.lambda., 3
N.sub.1 is the axial group refractive index and .lambda. is the wavelength. The function y was obtained from measurements made on bulk glass. The factor now found to be of significance involves the wavelength dependence of the refractive index.
For the particular system studied by Olshansky and Keck, for a TiO.sub.2 doped SiO.sub.2 graded fiber, the predicted deviation from the simple parabolic function is dramatic.
A waveguide alternative to the TiO.sub.2 doped silica fiber depends upon a lessening of refractive index by an outwardly increasing content of B.sub.2 O.sub.3 also in a high silica glass. This work is described, for example in U.S. Pat. No. 3,778,132. Preliminary studies have indicated that modal dispersion is not improved on the basis of measurements made on bulk glass samples of borosilicate glass following the Olshansky-Keck developed approach. In fact, it appears that modal dispersion is worsened as compared with fibers using the simple parabolic index gradient.